Formula 1 drivers said conditions in Sunday’s Qatar Grand Prix were “beyond the limit” and “too extreme”.
A number of drivers needed medical attention for either dehydration or heat exhaustion after a race run in high temperatures and humidity.
And Alpine driver Esteban Ocon said he had vomited in his helmet.
Mercedes driver George Russell said: “It was an absolutely brutal race, by far the most physical I have ever experienced. I felt close to fainting.”
Russell added he had “never experienced anything like that before”.
Williams driver Logan Sergeant retired because he could not cope with conditions any longer.
The team said he had later been “cleared by the medical team on-site after suffering from intense dehydration during the race (having been) weakened by having flu-like symptoms earlier in the week”.
The American’s team-mate Alex Albon was taken to the medical centre and treated for acute heat exposure. He, too, was assessed and cleared by the medical team.
Aston Martin’s Lance Stroll sought out an ambulance to receive treatment for dehydration and many drivers lay down in the garage of governing body the FIA after the race as they sought to recover.
The race was held in temperatures of more than 30C with high levels of humidity on a track with many high-speed corners.
The decision to limit stint lengths to just 18 laps because of safety concerns about the tyres exacerbated the situation as it meant drivers were pushing harder than usual. F1 grands prix have been generally defined by tyre management since Pirelli became the supplier in 2011.
Red Bull’s Max Verstappen, who won his third world title on Saturday and followed it up with victory in the grand prix, said: “These kind of temperatures are just too extreme.
“When I saw the weather before I came here, I wasn’t looking forward to it. It has nothing to do with more training; some of the guys struggling are extremely fit and probably fitter than me.
“The whole day you walk around (like you are) in the sauna and then in the night the humidity goes up.
“But there are a few places like that. Singapore is like a two-hour race and it’s very warm and it’s on the limit of what should be allowed. It was way too hot.”
McLaren’s Lando Norris, who finished third behind team-mate Oscar Piastri, said: “Today we probably found the limit. Sad we had to find it that way – some people ending up in the medical centre or passing out. A pretty dangerous thing.
“We are in a car that gets extremely hot, in a very physical race. On TV it probably doesn’t look that physical but when you have people who end up retiring it is too much; for the speeds we are doing it is too dangerous. It’s something we need to speak about because it shouldn’t have happened in the first place.”
Russell said: “I felt ill during this race. It was insane how hot it was. It was like you were inside an oven.
“I sometimes train in saunas and you push your body to the limit and you get to a point where it’s too hot and you’re like, ‘I want to get out.’ That was the feeling from about lap 12.
“There were points when I thought I was going to faint. It was unbelievable.”
Ocon said: “Lap 15-16, I was throwing up for two laps in the cockpit and I was like: ‘It is going to be a long race.’ Then I tried to remember the mental side is the strongest part of the body. I managed to get that under control. I was not expecting the race to be that hard.”
Ferrari’s Charles Leclerc said: “You can’t see as well. The reflexes are much slower; many things are happening in the body and it makes it more difficult to be precise at 200mph with the vision not as good as at the beginning.”
Leclerc was one of several drivers who expressed relief that the Qatar race has been scheduled for six weeks later next season, on 1 December.
But he added: “Even that I don’t know if it’s enough. It was at the limit of being on the dangerous side in the last few laps.
“We had time to rest (immediately after the race) and you could see many people were not feeling well.
“We will have to regroup and speak abut it to anticipate these things a bit.”